Chapter 5: Pollination and Fertilization
In 1898, in a laboratory in Kyiv, a botanist named Sergei Gavrilovich Nawaschin was cutting flower ovaries into slices thin enough for light to pass through. He stained the slices, mounted them on glass, and looked down his microscope at the tiny structures hidden inside the ovary of a lily, Lilium martagon, and of a fritillary, Fritillaria tenella.
Everyone at the time believed they knew what happened next inside a flower. A pollen grain landed on the stigma. It grew a tube down to the ovule. One male nucleus travelled down that tube and joined the egg. One sperm, one egg, one new plant — the same story as in animals.
But Nawaschin kept counting. Two male nuclei went down the tube, not one. One of them did join the egg. The other did not stop, did not die, and did not wander off. It moved deeper into the sac and fused with two other nuclei sitting in the middle of it.
Two fusions, in the same ovule, at the same time. At almost the same moment in France, Léon Guignard reported the same thing in lilies.
Flowering plants, it turned out, do something no animal does. The food store that feeds a germinating seed is not left over from the parent plant at all. It is fertilised too.
Diagram
Fig. 5.0-1 — Looking Inside A Lily Flower Line drawing of a whole Lilium martagon flower on the left with an inset on the right showing a thin longitudinal section through one ovule as seen under a light microscope; label petal, anther, stigma, style, ovary, ovule, and the inset as ‘thin stained section’.
5.1 Pollination
5.1.1 What Pollination Is
A flower is a plant’s reproductive shoot. Its stamens are the male organs, and each stamen has a stalk, the filament, carrying a swollen head called the anther. The carpel is the female organ, with a sticky tip called the stigma, a stalk called the style, and a swollen base called the ovary.
Inside the anther, a fine powdery material is produced. This powder is pollen, and it is not a dust at all. Under a lens it separates into countless separate particles called pollen grains, each one a single cell with a tough coat.
A pollen grain is the plant’s male gamete carrier. It carries within it the material that will later form two male gametes, and it also carries the food and machinery needed to make a delivery tube. What it cannot do is move on its own.
Why The Anther Is Not Enough
The egg of a flower sits deep inside the ovary, sealed under layers of tissue. The male gametes are up in the anther, sometimes on a completely different flower. Nothing can happen until the two are brought within reach of each other.
So the first step in making a fruit and its seeds is a journey. The pollen grain must leave the anther and arrive on a stigma. That arrival, and nothing more than that arrival, is what we call pollination.
Notice how little this definition claims. The pollen has landed; it has not yet joined anything. Fertilization comes later, and it is a separate event with a separate name, dealt with in Section 5.7.
5.1.2 The Three Routes Pollen Can Take
Pollen leaving an anther can arrive on a stigma by three different routes. They differ in one thing only: how far the pollen travels before it lands. The three are set out in FC 5.1 and drawn in Fig. 5.1-1.
The shortest route is no journey at all. Pollen from an anther falls straight onto the stigma of the very same flower. This is autogamy (auto: self, gamy: marriage).
A slightly longer route takes pollen to a different flower — but a flower borne on the same plant. This is geitonogamy (geitono: neighbouring). The pollen has crossed from one flower to another, yet it has never left the parent plant.
The longest route carries pollen away from the plant altogether, to the stigma of a flower on another plant of the same species. This is allogamy (allo: other). It is the transfer that wind, insects, water and other agents bring about.
Flowchart
FC 5.1 — The Three Routes Pollen Can Take Pollen released from anther → branch 1: falls on stigma of the same flower (autogamy); branch 2: carried to another flower on the same plant (geitonogamy); branch 3: carried to a flower on another plant of the same species (allogamy)
Diagram
Fig. 5.1-1 — The Three Routes Of Pollen Transfer Two flowering shoots side by side, labelled Plant A and Plant B, each bearing two open flowers; draw one arrow looping from an anther back to the stigma of the same flower (label autogamy), one arrow from a flower of Plant A to another flower of Plant A (label geitonogamy), and one arrow from a flower of Plant A to a flower of Plant B (label allogamy); bracket the first two arrows as self-pollination and the third as cross-pollination; label anther and stigma on one flower.
The first two routes keep the pollen on a single plant. The third carries it between plants. That single difference splits pollination into its two kinds, self-pollination and cross-pollination, which Sections 5.2 and 5.3 take up in turn.
5.1.3 Pollen Must Come From The Same Species
Pollination only works between plants of the same species. That sounds obvious until you look at where flowers actually grow.
An orchard may hold mango, guava and litchi trees, with wild trees such as neem, jamun or sheesham along its edge. Beyond the fence, fields of wheat, mustard, pulses and vegetables may all be in flower at once. Every one of them is shedding pollen into the same air.
The wind does not sort what it carries. It lifts pollen from all these plants together and drops it wherever it happens to fall. Insects are no more careful. Bees and butterflies visit whatever flower offers nectar, and they carry away whatever pollen sticks to them.
The result is that any one stigma receives a mixed load. Some of the grains on it belong to its own species. Most of them do not.
The Stigma As A Filter
What sorts the mixture is the stigma itself. Only pollen of the same species can respond to the stigma’s chemical signals, germinate on it, and grow a tube down towards the ovary. Pollen of every other kind fails to germinate and perishes there.
So the sorting is not done by the wind or by the insect. It is done at the last moment, on the stigma, by the plant itself. A flower can afford to receive the wrong pollen because the wrong pollen simply goes no further.
5.2 Self-Pollination
5.2.1 What Self-Pollination Is
Two of the three routes in FC 5.1 keep the pollen on one plant. Autogamy keeps it within one flower, and geitonogamy moves it between flowers of the same plant. Both are grouped together as self-pollination.
Grouping them together may look odd at first. In geitonogamy the pollen has clearly gone somewhere — it has left one flower and landed on another. Why is that still called self?
The answer lies in what a plant is made of. Every flower on a single plant has grown from the same original set of instructions, so all the flowers of one plant carry the same genetic characters. Moving pollen between them mixes nothing, because there is nothing different to mix.
5.2.2 Conditions That Allow Self-Pollination
Self-pollination cannot happen just anywhere. A flower has to be built and timed in a particular way for its own pollen to reach its own stigma.
Bisexuality
The first condition is that male and female parts must be within reach of one another. Most commonly this means a bisexual flower — one carrying both stamens and carpels in the same flower.
It can also happen in a plant with unisexual flowers, provided the male flowers and the female flowers are borne on the same plant. Pollen then has only to travel from one flower to another on that plant, which is geitonogamy.
Homogamy
The second condition is timing. Anthers ripen and split open at one moment; a stigma becomes receptive at another. If those two moments do not overlap, the pollen is shed before the stigma can accept it, or the stigma dries up before pollen is available.
Homogamy is the condition in which the anthers and the stigma of a flower mature at the same time. Without it, a bisexual flower can still fail to pollinate itself.
Flowers That Never Open
Some plants take no chances at all. Certain small bisexual flowers stay shut from bud to seed and never open, even when fully mature. Their anthers and stigma remain hidden inside, lying close against one another, so pollen has nowhere to go but onto the stigma beside it.
Such flowers are called cleistogamous flowers, and pea and pansy are common examples. No insect, no breath of wind and no water is needed, because nothing has to travel anywhere. Pollination here is not merely likely; it is unavoidable. Fig. 5.2-1 shows how the parts sit.
Diagram
Fig. 5.2-1 — A Cleistogamous Flower In Section Longitudinal section of a small closed bisexual flower with petals meeting over the reproductive parts; label sepal, unopened petals, anther touching stigma, stigma, style, ovary, ovule; add a short arrow from anther to stigma labelled ‘pollen transferred within the closed flower’.
5.2.3 Advantages Of Self-Pollination
Self-pollination has real benefits, and they all follow from the shortness of the journey.
- It is much surer. In a bisexual flower whose stamens and carpels mature together, the pollen has only millimetres to travel, and no outside help is needed.
- Parental characters are preserved indefinitely. Since the pollen and the egg come from one plant, the offspring carry the same characters generation after generation.
- There is no wastage of pollen. Almost every grain reaches a stigma, so even a small quantity will suffice.
- The flowers need not be large and showy. There is nobody to attract.
- Scent and nectar need not be produced at all.
The Economy Of A Short Journey
Read those five points together and one idea runs through all of them. Large petals, bright pigments, scent, nectar and vast quantities of pollen are all built from food the plant has made itself.
A self-pollinating plant does not have to build any of them. That saved nutrient material can go instead into leaves, roots, and the seeds themselves. Self-pollination is, above all, a great economy.
5.2.4 Disadvantages Of Self-Pollination
The same economy that makes self-pollination cheap also makes it narrow. Everything that reaches the next generation has come from one parent.
- Continued self-pollination, generation after generation, may lead to a weakening of the variety or of the species. The seeds produced are poor in quality and give rise to less vigorous offspring.
- Weaker or defective characters of the variety or breed cannot be eliminated. There is no second parent whose characters might mask or replace them.
- It does not yield new varieties. The genetic traits of the same plant pass to the next generation unchanged and unmixed, so there is little chance of improvement.
Why Sameness Becomes A Weakness
A plant that copies itself faithfully is well suited to the conditions its parent met. The difficulty is that conditions change. A new disease, a drier season or a warmer year can arrive within a single generation.
A population of identical plants meets such a change identically. If one plant cannot resist the disease, none of them can, because there is no variation in the population for the change to select from. Variation is the raw material of adaptation, and self-pollination produces almost none.
5.3 Cross-Pollination
5.3.1 What Cross-Pollination Is
The third route in FC 5.1 was allogamy — pollen carried away from the plant altogether. When pollen from the anthers of one plant reaches the stigma of a flower on a different plant of the same species, the process is called cross-pollination.
Two conditions are packed into that sentence, and both matter. The two plants must be separate individuals, and they must belong to the same species. Drop the first and it is self-pollination; drop the second and the pollen will not germinate at all.
Flowers That Open Wide
A flower that must send pollen to another plant cannot afford to stay shut. Its pollen has to be reachable, and its stigma has to be exposed to whatever arrives.
Large flowers with open, spreading petals and exposed reproductive structures are called chasmogamous flowers. Oxalis and Hibiscus are familiar examples. Because their pollen must travel some distance, they cannot manage without an outside carrier, and so they undergo mainly cross-pollination. Fig. 5.3-1 places such a flower beside the closed flower of Fig. 5.2-1.
Diagram
Fig. 5.3-1 — An Open Chasmogamous Flower Longitudinal section of a large open flower with petals spread wide apart; label sepal, spreading petal, filament, anther held above the petals, stigma exposed, style, ovary, ovule; add an arrow entering from outside the flower labelled ‘pollen arriving from another plant’.
5.3.2 Advantages Of Cross-Pollination
- The offspring are healthier. They receive characters from two different parents instead of one.
- The seeds produced are abundant and viable, so a larger proportion of them germinate and grow.
- New varieties may be produced, by cross-pollinating two different varieties of the same species or even two distinct species.
Where The Health Comes From
The first advantage is worth slowing down over, because it is the whole point of the process. In self-pollination, the male and female gametes come from one plant, so the offspring receives one set of characters twice over.
In cross-pollination they come from two plants, and no two plants of a species are exactly alike. The offspring therefore carries a fresh combination that neither parent had. A weakness carried by one parent can be masked by a strength carried by the other.
Repeat this across a whole population and the population becomes varied. When a disease or a drought arrives, some individuals in a varied population survive it — and it is their seed that carries the species forward.
5.3.3 Disadvantages Of Cross-Pollination
- The pollination is not always certain. Some pollinating agent is always needed, and that agent may or may not be available at the proper time.
- Pollen has to be produced in large quantity to ensure the maximum chance of pollination, which means a great wastage of pollen.
- The process is uneconomical for the plant, because the flowers have to be large, coloured and scented, and have to produce nectar to attract the pollinating agents.
Notice that the three disadvantages are the mirror image of the five advantages listed for self-pollination in Section 5.2.3. Everything the self-pollinating plant saved, the cross-pollinating plant must spend — and it still has no guarantee of success.
That is the bargain. Self-pollination is cheap and certain but produces sameness. Cross-pollination is costly and uncertain but produces variety.
5.3.4 Self-Pollination And Cross-Pollination Compared
The two kinds of pollination differ in where the pollen comes from, and every other difference follows from that one. Table 5.1 sets them side by side.
Read the last two rows together and you have the reason the rest of this chapter exists. A plant that can only self-pollinate has closed off its own future. Section 5.4 looks at the remarkable lengths to which flowering plants go to avoid exactly that.
5.4 How Nature Favours Cross-Pollination
The vast majority of flowering plants are cross-pollinated. That is a striking fact, because self-pollination is by far the cheaper and safer option, as Section 5.2 showed.
Plants pay the higher price because of what they get for it — variety, and the capacity to change. And they do not simply hope for cross-pollination. Flowers are built and timed in ways that make self-pollination difficult, unreliable or outright impossible.
Five such devices are described below and gathered together in FC 5.2. Each one works differently, but every one of them has the same effect: it keeps a flower’s own pollen from succeeding on its own stigma.
Flowchart
FC 5.2 — Devices That Favour Cross-Pollination Devices favouring cross-pollination → branch 1: unisexuality (male and female flowers separate) → branch 2: dichogamy (androecium and gynoecium mature at different times) → sub-branch 2a: protandry (anthers first); sub-branch 2b: protogyny (stigma first) → branch 3: self-sterility (own pollen fails to grow) → branch 4: herkogamy (mechanical barrier) → branch 5: heterostyly (anthers and stigma at different heights)
5.4.1 Unisexuality
The simplest way to prevent self-pollination is to keep the two sexes apart. A unisexual flower carries either stamens or carpels, never both.
Where a species bears its male and female flowers on separate plants, cross-pollination becomes the only possibility. Pollen made on a male plant has nowhere to go except to a flower on some other plant, because there is no stigma anywhere on the plant that produced it. Palms and papaya are examples of this arrangement.
Papaya usually has separate male and female trees. Less frequently, a hermaphrodite tree is found bearing both kinds of flowers.
Separation is not always so complete. In cucumber and gourd, male and female flowers are borne on the same plant. Autogamy is still impossible, since no single flower has both parts — but geitonogamy remains possible, so these plants are not fully protected from self-pollination. Fig. 5.4-1 shows the two kinds of flower.
Diagram
Fig. 5.4-1 — Male And Female Flowers Of A Cucurbit Two flowers of a cucumber plant drawn side by side in surface view on a common stem; on the left the male flower with stamens and a plain slender stalk, on the right the female flower with a stigma and a swollen ovary below the petals; label petal, stamen, anther, stigma, ovary, and mark the female flower’s ovary as ‘swollen — will become the fruit’.
5.4.2 Dichogamy
A flower may carry both sets of organs and still be unable to pollinate itself, if the two do not ripen together. Before going further, two collective names are worth having. The androecium is all the stamens of a flower taken together, and the gynoecium is all its carpels taken together.
Dichogamy is the condition in which the androecium and the gynoecium of the same flower mature at different times. In many bisexual flowers, the anthers and the stigma are simply never ready on the same day. It takes two forms.
Protandry
In some plants the anthers of a flower mature earlier than the stigma. This is protandry. By the time the flower’s own stigma becomes receptive, its pollen has already been shed and carried away. Lady finger, sweet pea, Salvia and sunflower show protandry.
Protogyny
In others the stigma of the flower matures earlier than the anthers. This is protogyny (protos: first, gyne: female). The stigma is receptive and waiting while the flower’s own anthers are still closed, so any pollen that arrives must have come from elsewhere. Custard apple and peepal show protogyny.
In either case, the flower’s own pollen and its own stigma are never available at the same moment. Cross-pollination becomes the only possibility.
5.4.3 Self-Sterility
Some plants block their own pollen after it has landed. Self-sterility is a condition in which the stigma may receive pollen from the anthers of the same flower, but that pollen fails to undergo any further growth.
The grain arrives, and then simply stops. It does not germinate into a tube, so it never reaches the ovary and never fertilises anything. Only pollen from another plant of the same species can effectively complete the process of setting seed.
This is a chemical device rather than a structural one. Nothing about the flower’s shape or timing prevents the pollen from arriving; the plant recognises its own pollen and refuses it. The ray florets of sunflower and many orchids behave in this way.
5.4.4 Herkogamy
In some flowers, the pollen of a flower simply cannot reach the stigma of that same flower, because something physically stands in the way. This is herkogamy (herkos: barrier), a mechanical or structural barrier between the anthers and the stigma.
The barrier can be very simple. In pansy and Iris, a hood-like flap grows over the stigma. Pollen falling from the flower’s own anthers lands on the outside of that hood and slides away without ever touching the receptive surface.
An insect pushing into the flower for nectar, however, passes under the hood from the front. It brushes the stigma with pollen already on its body, and only then collects fresh pollen on its way out.
5.4.5 Heterostyly
The last device separates the two organs in space. In heterostyly, the stigma and the anthers of a flower grow at different heights, which does not favour self-pollination.
A species showing heterostyly bears two kinds of flowers on different plants. One kind has a long style, so the stigma stands high while the anthers sit low down in the tube. The other kind has a short style, so the stigma sits low while the anthers stand high. Primrose is the classic example, and Oxalis shows the same arrangement. Both forms are drawn in Fig. 5.4-2.
Within a single flower, pollen falling from the anthers misses the stigma completely — it falls below it or above it. Between the two forms, however, the levels match perfectly. An insect entering a long-styled flower picks up pollen low on its body, and that is exactly the height at which the stigma of a short-styled flower is waiting.
Diagram
Fig. 5.4-2 — Heterostyly Longitudinal sections of two flowers of the same species side by side; A — long-styled flower with the stigma held high near the mouth of the corolla and the anthers attached low in the tube; B — short-styled flower with the stigma low in the tube and the anthers held high near the mouth; label petal, anther, stigma, style, ovary on each, and mark two horizontal dotted lines showing that the stigma of A lies at the level of the anthers of B and the stigma of B lies at the level of the anthers of A.
5.5 Agents Of Cross-Pollination
Cross-pollination needs a carrier, since pollen cannot move itself. Anything that regularly moves from flower to flower can serve as that carrier, and such a carrier is called an agent of pollination.
The two commonest agents are insects and wind. Some flowers are pollinated by other animals and by birds, such as squirrels and bats, and a few aquatic plants are pollinated by water itself.
No flower relies on all of these. Each category of flower carries special features that maximise its chances with its own particular agent — and those features are so consistent that you can usually tell, from the look of a flower alone, what carries its pollen.
5.5.1 Insect-Pollinated Flowers
Flowers pollinated by insects are called entomophilous flowers (entomon: insect, phile: affinity). Their features fall into two groups — those that advertise, and those that make pollen stick.
- The flowers are large.
- They are usually brightly coloured, to attract insects.
- They usually emit scent, again for attracting insects.
- They produce nectar, which is food for the insects.
- The pollen grains are sticky or spiny, so that they are carried on the insect’s body easily.
- The stigma is sticky, and does not generally hang out from the flower.
- The flowers tend to be borne in clusters, to make them conspicuous where the individual flowers are small, as in dahlia.
An insect does not visit a flower in order to pollinate it. It comes for the nectar, which is the plant’s payment. Colour and scent are the advertisement that brings the insect in; the sticky pollen and sticky stigma make sure that some pollen is transferred while the insect is busy feeding.
Notice also that the stigma stays tucked inside the flower. Pollen is being delivered to a precise spot on an insect’s body, so the stigma is placed exactly where that body will brush against it.
5.5.2 Wind-Pollinated Flowers
Flowers pollinated by wind are called anemophilous flowers (anemos: wind, phile: affinity). Wind cannot be attracted, so every feature that advertises has been abandoned, and every feature that helps a grain travel has been exaggerated.
- The flowers are small.
- They are usually not brightly coloured, and are often dull green.
- They do not produce scent or nectar.
- The stamens are long and hang out of the flower, so as to be exposed to the wind, as in maize.
- The anthers are large and loosely attached to the filaments, so that the slightest wind may move them. Such anthers are called versatile anthers.
- Pollen is produced in very large quantities.
- The pollen grains are light, dry and smooth, so that they can be carried away easily by the wind.
- The stigmas are feathery and hang out of the flower, to trap the pollen grains.
Every one of these features answers the same problem. Wind carries pollen everywhere and delivers it nowhere in particular, so the chance of any one grain reaching a stigma is very small. The plant compensates by releasing enormous numbers of grains and by making its stigmas as easy to hit as possible. Fig. 5.5-1 places the two kinds of flower side by side.
Diagram
Fig. 5.5-1 — An Insect-Pollinated Flower And A Wind-Pollinated Flower Two flowers drawn side by side to the same scale; A — a large insect-pollinated flower with broad spreading petals, anthers and stigma held within the flower and nectar at the base; label petal, nectary, sticky pollen on anther, sticky stigma inside corolla. B — a small dull flower of a grass with no showy petals, long filaments hanging outside and a large branched stigma; label reduced bract, long filament, versatile anther hanging out, feathery stigma hanging out.
5.5.3 Wind-Pollinated And Insect-Pollinated Flowers Compared
The two sets of features are set out feature by feature in Table 5.2. Read it as two different solutions to one problem, not as two lists to be learnt.
5.5.4 Water-Pollinated Flowers
Flowers pollinated by water are called hydrophilous flowers (hydro: water, phile: affinity), and they are found only in aquatic plants.
- Pollen grains are produced in large numbers.
- In some plants the pollen grains have a specific gravity almost equal to that of water, so that they remain floating below the surface of the water.
- In some special cases, the male flowers themselves are such that they float on the surface of the water until they meet the female flowers.
The second feature is worth pausing on. Specific gravity compares the density of a substance with the density of water. A grain much denser than water would sink to the bottom and be lost; a much lighter one would be stranded on the surface. A grain of almost the same density does neither, and drifts at the level where the female flowers are waiting.
Vallisneria shows the third arrangement, drawn in Fig. 5.5-2. Its male flowers break free from the parent plant and float on the surface. The female flowers are carried up to the surface on long coiled stalks, and the drifting male flowers reach them there.
Diagram
Fig. 5.5-2 — Water Pollination In Vallisneria Cutaway view of a pond with the water surface drawn as a horizontal line; on the left a male plant rooted in the mud with a cluster of tiny male flowers at its base and several detached male flowers floating on the surface; on the right a female plant rooted in the mud with a long coiled stalk carrying a single female flower up to the surface; label male plant, female plant, floating male flowers, female flower, long coiled stalk, developing fruit, water surface.
5.5.5 Pollination By Birds And Other Animals
Some flowers are pollinated by birds, a condition called ornithophily (ornitho: bird). Bignonia and Canna are examples. Bird-pollinated flowers are usually strongly built, so that a probing beak does not tear them, and they hold plenty of thin nectar.
Mammals pollinate flowers too. Bats visit large flowers that open at night and carry pollen on their fur; the durian, Durio zibethinus, and the baobab, Adansonia digitata, are both bat-pollinated. Squirrels and other small mammals that feed at flowers carry pollen on their fur in the same way.
Rafflesia, which produces the largest single flowers in the world, shows how far this can go. Its flowers sit at ground level, have no stem or leaves of their own, and give off a strong smell of decaying flesh. That smell attracts carrion flies, which are its pollinators; they arrive expecting rotting meat and leave carrying pollen.
5.5.6 Artificial Pollination
When a human being transfers pollen to a stigma, the process is called artificial pollination.
It is an old practice. In the ancient civilization of Babylonia it was common to sprinkle the ‘male flowers’ of palms over the ‘female’ flowers to make sure the trees set a crop of dates. Those farmers knew nothing about the sexuality of palm flowers, and nothing about pollination. They only knew that the trees bore better when they did it.
These days artificial pollination, also called artificial crossing, is a standard practice of plant breeders and scientists working to evolve new varieties. The aim is complete control over which two plants become the parents, and the procedure set out in FC 5.3 achieves it.
The anthers are removed from young flowers before any pollen is shed, a step called emasculation. Those flowers are then covered with plastic bags, a step called bagging, so that no stray pollen can reach them. Later the breeder opens the bag and pollinates the flowers by hand with pollen from plants of the desired variety.
Flowchart
FC 5.3 — Steps In Artificial Crossing Select the two parent plants → remove the anthers from young unopened flowers of the chosen female parent (emasculation) → cover those flowers with plastic bags (bagging) → collect pollen from the chosen male parent → dust it on the stigma by hand when the stigma is receptive → bag the flower again → allow the fruit and seeds to develop → raise the seeds and study the new variety
5.6 Some Examples Of Pollination
Sections 5.5.1 and 5.5.2 listed the features of insect-pollinated and wind-pollinated flowers. Two familiar plants now show those features actually at work.
5.6.1 Insect-Pollination In Sweet Pea
The flower of sweet pea, Lathyrus odoratus, has an unusual shape, and the shape is a mechanism. Its five petals are not alike. One large petal stands upright at the back and is called the standard petal. Two petals spread sideways as the wing petals. The remaining two are joined together below into a boat-shaped structure, the keel petal, and the stamens and the carpel lie hidden inside that keel. The parts are drawn in Fig. 5.6-1.
An insect such as a bee is drawn in by the conspicuous corolla and alights on it. It thrusts its long tongue deep into the flower in search of nectar.
In the struggle to reach the nectar, the bee’s weight presses the wing petals down. The wing petals are joined to the keel, so the keel is forced down with them, and the stamens and stigma inside it are pushed out against the underside of the insect’s body.
If the bee has already visited another sweet pea flower, pollen from that flower is sticking to its underside. That pollen is now pressed onto the stigma of this flower, and cross-pollination is complete. As the bee flies away, it carries fresh pollen from this flower’s anthers on the same part of its body, ready for the next flower it visits.
When The Bee Does Not Come
Cross-pollination in sweet pea depends entirely on a visitor, and a visitor may not arrive. If it does not, the flower is still not lost.
The stamens and the carpel are enclosed together within the keel, and they stand at more or less the same level. Pollen shed inside that closed space has very little distance to fall and very few places to fall other than the stigma beside it. In sweet pea, therefore, if cross-pollination does not take place, there is every chance of self-pollination instead.
Diagram
Fig. 5.6-1 — Floral Parts Of Sweet Pea Seven-part plate. A — whole flower in side view, labelling sepal, standard petal, wing petal, keel petal. B — standard petal removed and shown separately. C — the two wing petals shown separately. D — the keel petal shown separately. E — flower with the petals removed, labelling sepals, staminal tube of nine joined stamens, free stamen, anthers, style, stigma. F — the carpel alone, labelling ovary, style, stigma. G — a single pollen grain drawn much enlarged.
5.6.2 Wind-Pollination In Maize
Maize, Zea mays, is pollinated entirely by wind, and it is a good plant to study because its male and female flowers are not only separate but sit at opposite ends of the plant. Fig. 5.6-2 shows the whole plant.
A group of flowers borne together on a common stalk is called an inflorescence. In maize the male inflorescence forms a loose branched plume at the very top of the stem, called the tassel. The female inflorescence is the cob, borne lower down in the angle of a leaf, and from its tip hangs a mass of long silky threads. Each of those threads is the style of one female flower, and its whole length is receptive to pollen.
Why Height And Looseness Matter
The anthers hang outward from the tassel, and they are loosely attached to their filaments. The slightest movement of air shakes them, so the pollen is shed easily and freely, and it is shed from the highest point on the plant, where the wind is strongest and least obstructed by leaves.
The pollen is then simply blown away. Some of it drifts onto the silks of a female flower, which present an enormous receptive surface for exactly this purpose — a whole head of long threads waiting in the moving air.
There is one more safeguard. The male flowers of the tassel mature earlier than the female flowers of the same plant. By the time a plant’s own silks are ready to receive, its own tassel has largely finished shedding, so there is less chance of self-pollination and a greater chance that the pollen arriving has come from a neighbouring plant.
Diagram
Fig. 5.6-2 — The Maize Plant Whole maize plant drawn from the side; label male inflorescence (tassel) at the top of the stem with hanging anthers, female inflorescence (cob) borne in the axil of a leaf lower down, styles (silk) emerging from the tip of the cob, leaf, stem, and prop roots at the base; show fine pollen being blown from the tassel across to the silks.
5.7 Fertilization
Pollination ends the moment a pollen grain settles on a stigma. Nothing has yet joined anything. The male gametes are still sealed inside a grain on the outside of the carpel, and the egg is still deep inside the ovary.
Fertilization is the event that follows: the union or fusion of the nuclei of the male and the female gametes. A gamete is a reproductive cell that carries half the normal number of chromosomes, so that two gametes fusing restore the full number.
In flowering plants the arrangement is this. The pollen grain produces two male gametes. The ovule inside the ovary produces the female gamete. Everything in this section is about how the first reaches the second.
5.7.1 The Pollen Grain
A mature pollen grain is a single cell, and it is unusually well armoured for one. It has a double wall, shown in Fig. 5.7-1. The outer wall is the exine, which is thick, tough and often sculptured, and it is what allows a grain to survive a journey through dry air. The inner wall is the intine, which is thin and delicate.
The exine is not continuous. It is interrupted at one or more thin spots called germ pores, and these are the only places where anything can grow out of the grain.
Inside, the nucleus of the grain has already divided before the pollen is shed. One of the two nuclei produced is the tube nucleus, which will control the growth of the delivery tube. The other is the generative nucleus, also called the male nucleus, and it is the one that will eventually give rise to the gametes.
It is at this stage — two nuclei, one tough double wall — that the grain is transferred to a stigma. That transfer is the pollination of Sections 5.1 to 5.6. Nothing further happens to the grain unless it has landed on the stigma of a plant of its own species.
Diagram
Fig. 5.7-1 — A Mature Pollen Grain A single pollen grain drawn much enlarged in section, roughly oval with a sculptured outer surface; label exine (thick outer wall), intine (thin inner wall), germ pore, tube nucleus, generative nucleus, cytoplasm.
5.7.2 The Ovule And The Embryo Sac
The ovule is a small body borne inside the ovary, attached to its wall. It is where the female gamete is made, and it is destined to become the seed, just as the ovary itself is destined to become the fruit containing that seed.
The number of ovules decides the number of seeds. A single ovule produces a single-seeded fruit; many ovules produce a many-seeded fruit.
The Coverings And The Opening
Each ovule has one or two protective coverings called integuments, which wrap it almost completely. Almost, but not quite. At one end the integuments stop short of meeting, leaving a small opening called the micropyle.
That gap is not an accident of construction. It is the door through which the pollen tube will later enter, and without it the tube would have to dissolve its way through the coverings.
Enclosed by the integuments is the nucellus, a mass of food-laden cells. Deeper still, inside the nucellus, lies the embryo sac, and the female gamete is one of the cells within it. Fig. 5.7-2 shows the arrangement.
The Seven Cells Of The Embryo Sac
The embryo sac contains seven cells, arranged as three, three and one.
Three cells lie at the micropylar end — the end nearest the opening. One of these is the egg cell, the female gamete itself. The other two are the synergids, which flank it and help guide the incoming pollen tube.
Three more cells lie at the opposite end and are called the antipodal cells. They take no part in fertilization.
The seventh is one large central cell, occupying the middle of the sac. It contains two nuclei, called the polar nuclei, lying fused together. Keep these two in mind — they matter greatly in Section 5.7.4.
Diagram
Fig. 5.7-2 — Longitudinal Section Of An Ovule Longitudinal section of an ovule attached to the ovary wall; label ovary wall, funicle, micropyle, outer integument, inner integument, nucellus, embryo sac, egg cell, two synergids at the micropylar end, three antipodal cells at the opposite end, large central cell with two polar nuclei.
5.7.3 Germination Of The Pollen Grain
A pollen grain germinates only if it falls on the stigma of a plant of its own species. On any other stigma it disintegrates, which is the filtering met in Section 5.1.3.
On the right stigma, the grain is stimulated to germinate by sugars secreted by the stigma itself. The intine bulges out through a germ pore in the exine, and a slender pollen tube begins to grow out of the grain.
Three Nuclei In One Tube
The tube grows downward carrying at its tip the generative nucleus and the tube nucleus. As it grows, the generative nucleus divides into two sperm nuclei, also called male gamete nuclei.
So a germinating pollen grain now holds three nuclei — two sperm nuclei and one tube nucleus. They are not separated from each other by any cell walls; all three share a single common cytoplasm within the tube.
The Journey Down The Style
The pollen tube does not push its way between the tissues of the style. It digests them. It secretes enzymes that dissolve the tissues ahead of it, and grows down through the stigma and the style along the channel it has made, until it reaches the ovary.
There it turns towards an ovule, pushes through the micropyle, and reaches the embryo sac. The whole path is shown in Fig. 5.7-3.
The tube nucleus has directed the growth of the tube all the way down. Its work is now finished, and it disintegrates.
Diagram
Fig. 5.7-3 — Growth Of The Pollen Tube Longitudinal section of a complete carpel with pollen grains germinating on the stigma; label stigma, germinating pollen grain, pollen tube growing down through the style, style, ovary wall, ovule, micropyle, embryo sac, and the tip of the pollen tube entering the micropyle; mark on the tube the two sperm nuclei and the tube nucleus ahead of them.
5.7.4 Double Fertilization And Triple Fusion
Two sperm nuclei have now been released into the embryo sac, and both are used. This is where flowering plants do something no animal does.
The first sperm nucleus fuses with the nucleus of the egg cell. The product is the zygote, the single cell from which the whole new plant will grow. That is fertilization in the ordinary sense.
The second sperm nucleus does not stop. It moves to the centre of the sac, to the two polar nuclei of the central cell. Those two have united to form a single secondary nucleus, and the sperm nucleus fuses with it.
Three nuclei have therefore joined in this second fusion — two polar nuclei and one sperm nucleus. For that reason it is called triple fusion, and the product is the endosperm nucleus, from which the food store of the seed will develop.
Two separate fusions have occurred inside one embryo sac, and this is why the whole process is called double fertilization. FC 5.4 sets out the events in order.
Flowchart
FC 5.4 — Events Of Double Fertilization Pollen tube enters one synergid and releases two sperm nuclei → branch 1: first sperm nucleus fuses with the egg cell nucleus → zygote → embryo; branch 2: two polar nuclei unite to form the secondary nucleus → second sperm nucleus fuses with the secondary nucleus (triple fusion) → endosperm nucleus → endosperm; synergids and antipodal cells degenerate
Diagram
Fig. 5.7-4 — Double Fertilization In The Embryo Sac Enlarged view of an embryo sac with the tip of the pollen tube entering through the micropyle and discharging into one synergid; label pollen tube, two sperm nuclei, degenerating synergid, second synergid, egg cell, secondary nucleus in the central cell, antipodal cells; draw one arrow from a sperm nucleus to the egg cell labelled ‘first fertilization — forms the zygote’ and a second arrow from the other sperm nucleus to the secondary nucleus labelled ‘second fertilization, triple fusion — forms the endosperm nucleus’.
5.7.5 Fate Of Floral Parts After Fertilization
Once fertilization is over, the flower has served its purpose. Petals, scent and nectar were advertisements to a pollinator, and the pollinator is no longer needed.
The petals, stamens, style and stigma wither and generally fall off. The calyx behaves in either of two ways. In many plants it falls off; in others it remains attached in a dried and shrivelled form, and apple and guava clearly show such dried sepals. In brinjal, the green calyx persists and stays attached even to the ripe fruit.
Meanwhile the ovary does the opposite of withering. It enlarges to form the fruit, its wall becoming the fruit wall, which may be dry and hard in some plants and fleshy in others. The ovules within it become the seeds.
The fruit thus contains one or more seeds, developed from one or more ovules attached to the ovary by means of a placenta. When the fruit is ripe, the seeds inside are released by one method or another, and in suitable conditions each grows into a new plant. FC 5.5 gathers the changes together.
Flowchart
FC 5.5 — Fate Of Floral Parts After Fertilization After fertilization → ovary → fruit; ovary wall → pericarp (the fruit wall); ovule → seed; funicle → stalk of the seed; outer integument → testa and inner integument → tegmen, together forming the seed coat; secondary nucleus (after triple fusion) → endosperm; zygote formed from the egg cell → embryo; synergids and antipodal cells → degenerate and disorganise; petals, stamens, style and stigma → wither and fall off; calyx → falls off, or persists in dried form
5.8 Summary
- Pollen is produced by the anthers of the stamens and consists of separate pollen grains, each a single cell carrying the material that will form two male gametes. Pollen cannot move by itself.
- Pollination is the transfer of pollen grains from the anther of a flower to the stigma of a flower. It is the arrival of the pollen only, not its fusion with anything.
- Pollen can reach a stigma by three routes — autogamy (same flower), geitonogamy (another flower of the same plant) and allogamy (a flower on another plant of the same species).
- Pollination works only between plants of the same species. A stigma receives a mixed load of pollen, but only pollen of its own species can germinate on it; the rest perishes there.
- Self-pollination covers autogamy and geitonogamy, because all the flowers of one plant carry the same genetic characters.
- Self-pollination requires bisexuality — male and female parts within reach — and homogamy, in which the anthers and the stigma mature at the same time.
- Cleistogamous flowers never open even at maturity, so their anthers and stigma lie together inside and self-pollination is certain. Pea and pansy are examples.
- Self-pollination is surer, wastes no pollen, and needs neither large showy flowers nor scent and nectar. It preserves parental characters indefinitely and is a great economy for the plant.
- Continued self-pollination weakens the variety or species, cannot eliminate defective characters, and yields no new varieties, because it produces no variation.
- Cross-pollination is the transfer of pollen from the anthers of one plant to the stigma of a flower on another plant of the same species. Chasmogamous flowers, which open wide and expose their reproductive parts, undergo mainly cross-pollination.
- Cross-pollination gives healthier offspring, abundant viable seeds, and the possibility of new varieties, but it is uncertain, wasteful of pollen and uneconomical for the plant.
- The vast majority of flowering plants are cross-pollinated, and several devices favour this: unisexuality, dichogamy, self-sterility, herkogamy and heterostyly.
- Dichogamy is the maturing of the androecium and gynoecium at different times — protandry when the anthers ripen first, protogyny when the stigma ripens first.
- Insect-pollinated flowers are large, brightly coloured and scented, secrete nectar, and have sticky or spiny pollen and a sticky stigma held within the flower.
- Wind-pollinated flowers are small and dull, without scent or nectar, with long hanging stamens, versatile anthers, feathery projecting stigmas, and very large quantities of light dry smooth pollen.
- Water-pollinated flowers occur only in aquatic plants. Their pollen is produced in large numbers, and may float at the level of the stigmas or, as in Vallisneria, be carried by whole floating male flowers.
- Some flowers are pollinated by birds (ornithophily), and others by bats, squirrels and other animals.
- In artificial pollination the breeder removes the anthers of a young flower (emasculation), covers it (bagging), and later dusts on pollen from a chosen plant, so producing new varieties deliberately.
- Fertilization is the fusion of the nuclei of the male and female gametes. The pollen grain produces two male gametes; the ovule inside the ovary produces the female gamete.
- A mature pollen grain has a thick outer exine with germ pores and a thin inner intine, and contains a tube nucleus and a generative nucleus.
- An ovule is covered by integuments that leave an opening, the micropyle, and encloses the nucellus, within which lies the embryo sac.
- The embryo sac contains seven cells — an egg cell with two synergids at the micropylar end, three antipodal cells at the other end, and one large central cell holding two polar nuclei.
- A pollen grain germinates only on a stigma of its own species. A pollen tube grows out through a germ pore, the generative nucleus divides into two sperm nuclei, and the tube digests its way down the style to the ovary.
- The tube enters the ovule through the micropyle and a synergid, and releases its two sperm nuclei. The tube nucleus then disintegrates.
- One sperm nucleus fuses with the egg cell nucleus to form the zygote. The other fuses with the secondary nucleus formed by the two polar nuclei — a triple fusion — to form the endosperm nucleus. Two fusions in one embryo sac make this double fertilization.
- After fertilization the petals, stamens, style and stigma wither and fall, while the calyx may fall or persist in dried form.
- The ovary enlarges into the fruit and its wall becomes the pericarp; the ovules become the seeds, with the integuments forming the testa and tegmen, the zygote forming the embryo, and the endosperm nucleus forming the endosperm.
Flowchart
FC 5.6 — From Pollen Grain To New Plant Anther sheds pollen → pollen carried by an agent, or falls within the same flower → pollen lands on a stigma of the same species → grain germinates and a pollen tube grows down the style → generative nucleus divides into two sperm nuclei → tube enters the ovule through the micropyle → sperm nucleus 1 fuses with the egg cell → zygote → embryo; sperm nucleus 2 fuses with the secondary nucleus (triple fusion) → endosperm nucleus → endosperm → ovule becomes the seed and the ovary becomes the fruit → seed released from the ripe fruit → seed germinates into a new plant → the new plant flowers, and the cycle returns to the anther shedding pollen
5.9 Key Terms
- Allogamy — the falling of pollen on the stigma of a flower borne on another plant of the same species.
- Androecium — all the stamens of a flower taken together.
- Anemophilous — pollinated by wind.
- Antipodal cells — the three cells at the end of the embryo sac away from the micropyle, which take no part in fertilization.
- Artificial pollination — the transfer of pollen to a stigma by a human being.
- Autogamy — the falling of pollen from an anther onto the stigma of the same flower.
- Bagging — covering an emasculated flower so that no unwanted pollen can reach its stigma.
- Central cell — the single large cell in the middle of the embryo sac, containing the two polar nuclei.
- Chasmogamous — describing a flower that opens wide and exposes its reproductive parts.
- Cleistogamous — describing a small flower that never opens, so that its own anthers pollinate its own stigma inside.
- Cross-pollination — the transfer of pollen from the anthers of one plant to the stigma of a flower on another plant of the same species.
- Dichogamy — the maturing of the androecium and the gynoecium of a flower at different times.
- Double fertilization — the occurrence of two fusions in one embryo sac, one forming the zygote and the other the endosperm nucleus.
- Egg cell — the female gamete, lying at the micropylar end of the embryo sac.
- Emasculation — the removal of the anthers from a young flower before its pollen is shed.
- Embryo sac — the structure inside the nucellus of an ovule that contains the egg cell and six other cells.
- Endosperm nucleus — the nucleus formed by triple fusion, from which the food store of the seed develops.
- Entomophilous — pollinated by insects.
- Exine — the thick, tough, often sculptured outer wall of a pollen grain.
- Fertilization — the union of the nuclei of a male and a female gamete.
- Geitonogamy — the falling of pollen on the stigma of another flower borne on the same plant.
- Generative nucleus — the nucleus of a pollen grain that divides to form the two sperm nuclei.
- Germ pore — a thin spot in the exine through which the pollen tube grows out.
- Gynoecium — all the carpels of a flower taken together.
- Herkogamy — a mechanical or structural barrier that prevents a flower’s pollen from reaching its own stigma.
- Heterostyly — the growth of the stigma and the anthers at different heights, which discourages self-pollination.
- Homogamy — the maturing of the anthers and the stigma of a flower at the same time.
- Hydrophilous — pollinated by water.
- Integuments — the one or two protective coverings of an ovule.
- Intine — the thin, delicate inner wall of a pollen grain.
- Micropyle — the small opening left by the integuments, through which the pollen tube enters the ovule.
- Nectar — the sugary liquid secreted by a flower as food for its pollinator.
- Nucellus — the mass of food-laden cells within the integuments of an ovule, enclosing the embryo sac.
- Ornithophily — pollination by birds.
- Ovule — the body inside the ovary that contains the embryo sac and becomes the seed.
- Polar nuclei — the two nuclei of the central cell, which unite to form the secondary nucleus.
- Pollen — the powdery material produced by the anthers of a flower.
- Pollen grain — a single particle of pollen, a cell with a double wall that carries the male gametes.
- Pollen tube — the tube that grows out of a germinating pollen grain and carries the sperm nuclei to the embryo sac.
- Pollination — the transfer of pollen grains from the anther of a flower to the stigma of a flower.
- Protandry — dichogamy in which the anthers mature earlier than the stigma.
- Protogyny — dichogamy in which the stigma matures earlier than the anthers.
- Secondary nucleus — the nucleus formed when the two polar nuclei unite in the central cell.
- Self-pollination — the transfer of pollen to the stigma of the same flower or of another flower on the same plant.
- Self-sterility — the condition in which a flower’s own pollen lands on its stigma but fails to grow further.
- Sperm nuclei — the two male gamete nuclei formed by division of the generative nucleus.
- Synergids — the two cells flanking the egg cell, which help guide the pollen tube into the embryo sac.
- Triple fusion — the fusion of one sperm nucleus with the two polar nuclei, forming the endosperm nucleus.
- Tube nucleus — the nucleus of a pollen grain that directs the growth of the pollen tube and then disintegrates.
- Unisexual — describing a flower that bears either stamens or carpels, but not both.
- Versatile anthers — large anthers loosely attached to their filaments, so that the slightest wind shakes them.
- Zygote — the cell formed by the fusion of a sperm nucleus with the egg cell nucleus, which develops into the embryo.
5.10 Points To Ponder
Self-pollination is described as an economy and cross-pollination as an expense, yet the vast majority of flowering plants are cross-pollinated. What does that tell you about which of the two is really the more valuable to a species over many generations?
Cleistogamous flowers guarantee pollination, and chasmogamous flowers guarantee nothing. Some species produce both kinds on the same plant in different seasons. What might a plant gain by keeping both options open?
Herkogamy uses a barrier, dichogamy uses timing, heterostyly uses height and self-sterility uses chemistry — yet all four achieve the same result. Why might so many different solutions have arisen to a single problem, instead of one best solution spreading everywhere?
Wind-pollinated plants waste almost all their pollen, while insect-pollinated plants waste very little. Yet wind pollination has not disappeared. Under what conditions would giving up colour, scent and nectar be the better bargain?
A flower’s petals, scent and nectar all exist for a visitor rather than for the plant itself. In what sense, then, is an insect part of a flowering plant’s reproductive process rather than merely a passer-by?
Flowering plants build the seed’s food store only after a second fertilization, while many other plants stock their seeds in advance. What difference does that timing make to a plant whose flowers are not all successfully pollinated?
5.11 Objective Questions
Multiple Choice
- The transfer of pollen from an anther to the stigma of another flower borne on the same plant is called: (1 mark)
- autogamy
- geitonogamy
- allogamy
- herkogamy
- Which of the following is a feature of a wind-pollinated flower? (1 mark)
- Sticky, spiny pollen grains
- Nectar secreted at the base of the corolla
- Feathery stigma hanging out of the flower
- Brightly coloured petals borne in clusters
- In a flower showing protogyny: (1 mark)
- the anthers mature before the stigma
- the stigma matures before the anthers
- the anthers and stigma mature together
- the flower never opens at all
- The pollen tube enters the ovule through the: (1 mark)
- nucellus
- micropyle
- germ pore
- funicle
- Triple fusion involves: (1 mark)
- one sperm nucleus and the egg cell nucleus
- two sperm nuclei and one polar nucleus
- one sperm nucleus and the two polar nuclei
- the tube nucleus and the two synergids
Assertion And Reason
- In each of the following, an Assertion (A) is followed by a Reason (R). Select the correct option.
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
- Assertion (A): Cleistogamous flowers do not need any pollinating agent. Reason (R): Their anthers and stigma lie close together inside a flower that never opens. (1 mark)
- Assertion (A): Wind-pollinated plants produce enormous quantities of pollen. Reason (R): Wind carries pollen at random, so most of the grains never reach a stigma. (1 mark)
Match The Following
- Match the items in Column I with those in Column II. (4 marks)
| Column I | Column II |
|---|---|
| a. Versatile anthers | i. Pollination by birds |
| b. Vallisneria | ii. Barrier over the stigma |
| c. Ornithophily | iii. Wind pollination |
| d. Herkogamy | iv. Floating male flowers |
Fill In The Blanks
- Complete the following statements. (5 marks)
- The outer wall of a pollen grain is called the ………. .
- Flowers that open wide and expose their reproductive parts are called ………. flowers.
- The removal of anthers from a young flower by a breeder is called ………. .
- The two nuclei of the central cell of the embryo sac are the ………. nuclei.
- After fertilization the ovary wall becomes the ………. of the fruit.
True Or False
- State whether each statement is true or false. (4 marks)
- Pollination and fertilization are two names for the same event.
- A pollen grain germinates on the stigma of any flower it lands on.
- Antipodal cells take no part in fertilization.
- In brinjal the calyx remains attached to the ripe fruit.
Name The Following
- Name the following. (5 marks)
- The condition in which the anthers and stigma of a flower mature at the same time.
- The mass of food-laden cells enclosed by the integuments of an ovule.
- The nucleus that directs the growth of the pollen tube and then disintegrates.
- The device by which the stigma and anthers of a flower grow at different heights.
- The two cells that flank the egg cell in the embryo sac.
Odd One Out
- Pick the odd one out in each group and give a reason for your choice. (3 marks)
- Exine, intine, germ pore, micropyle
- Protandry, protogyny, homogamy, dichogamy
- Maize, grass, Dahlia, wheat
5.12 Check Your Concepts
Define pollination. (2 marks)
State any two advantages of self-pollination to a plant. (2 marks)
Give a reason why geitonogamy is treated as self-pollination even though the pollen moves from one flower to another. (2 marks)
Name the two products of double fertilization and state what each one develops into. (2 marks)
Explain, with an example, how unisexuality favours cross-pollination. (3 marks)
Why does a stigma receive pollen of many species but allow only one kind to grow? (3 marks)
Describe the three steps by which a plant breeder carries out artificial crossing, and state the purpose of each step. (3 marks)
Differentiate between self-pollination and cross-pollination, stating the basis of difference in each case. Give any four differences. (4 marks)
Differentiate between wind-pollinated and insect-pollinated flowers, stating the basis of difference in each case. Give any four differences. (4 marks)
Describe the structure of a mature embryo sac, and explain what happens to each of its parts after the pollen tube discharges into it. (5 marks)
What would happen to a species of flowering plant if every one of its flowers became cleistogamous? Explain your answer using what you know about variation. (5 marks)
5.13 Figure-Based Questions
- Study Fig. 5.4-2, which shows two flowers of the same species. (4 marks)
- Name the device illustrated by the two flowers.
- State one difference in the position of the stigma between flower A and flower B.
- Explain how this arrangement discourages self-pollination.
- Name one plant that shows this device.
- Refer to Fig. 5.7-2, a longitudinal section of an ovule. (5 marks)
- Name the structure that the ovule as a whole will become after fertilization.
- Name the opening left by the integuments and state its use.
- How many cells does the embryo sac contain, and how are they arranged?
- State one function of the synergids.
- Redraw the embryo sac alone and label the egg cell, the synergids, the antipodal cells and the polar nuclei.
- Fig. 5.6-1 shows the floral parts of sweet pea. (4 marks)
- Name the petals labelled as the wing petals and the keel petal.
- Which floral parts lie enclosed within the keel?
- Explain how the visit of a bee brings about pollination in this flower.
- Give a reason why self-pollination is likely in sweet pea if no insect visits.
- Refer to Fig. 5.5-2, which shows a wind-pollinated flower. (3 marks)
- Name the process the figure represents.
- State one function of the feathery stigma shown in the figure.
- Give a reason why the anthers in such a flower are loosely attached to their filaments.